Having done some reading into boiler fitted with weather compensation I can
sort of see the logic of reducing the flow temp as outside temp increases.

I also see that a boiler should only be fired where it can acheive a 10deg
increase in temp to prevent short cycling (rapid on and off to keep the flow
temp around the desired temp)

This is find when the flow is say 80degC, to fire the boiler once the return
gets to below 70degC and heat until 80deg.

The problem occurs at lower temps. If the boiler requires a flow of just
25deg, (say outside is 20deg), you cannot acheive the 10deg differencial so
the boiler would not fire. How do the commercial weather compensators cope
with this? or do they increase the ammount of short cycling?



Example of Micromat:

On the Micromat, the CH flow temperature can be set to one of three preset
levels - 50, 70 and 85 degrees. At low temperatures, say -1,
the boiler will run at its preset temperature. However, at higher
temperatures outside, less heat will be needed, until at 20 degrees
outside, nothing is required.. The end points of the graph can be adjusted
as appropriate. In practice, there are various effects -
the main ones being that at switch on, the boiler doesn't automatically go
to full power, and that the power is starting to increase as it's getting
colder outside